1. Field of the Invention
The present invention relates generally to switch-mode amplifiers. More particularly, the present invention relates to a full-bridge amplifier that can recover all reactive energy. The present invention is useful in induction heating and plasma applications, but by no means is limited to such applications.
2. Related Art
Induction heating is a method of heating electrically conductive materials such as metals. Induction heating relies on, as the name implies, inducing electrical currents within a material to be heated. These induced currents, called eddy currents, dissipate energy and bring about heating. Common uses of induction heating include: heat treating, welding, and melting.
Prior to the development of induction heating, gas and oil-fired furnaces provided the prime means of heating metals and nonmetals. The advantages that induction heating offers over furnace techniques are numerous. For example, greater heating rates can be achieved by induction heating than can be achieved by gas or oil furnaces. Higher heating rates lead to shorter heating times, which lead to productivity increases and reduced labor costs. Furthermore, given today's environmental concerns, induction heating is an attractive alternative to pollution producing furnaces.
The basic components of an induction heating system are a tank circuit having an inductor coil, an RF power supply, and the material to be heated -i.e., the work piece. The work piece is heated by placing the work piece within the inductor coil of the tank circuit and applying a high-power, RF alternating voltage to the tank circuit using the RF power supply. The RF voltage applied to the tank circuit causes an alternating current to flow through the inductor coil. The flow of an alternating current through the inductor coil generates an alternating magnetic field that cuts through the work piece placed in the inductor coil. It is this alternating magnetic field that induces the eddy currents that heat the work piece.
The work piece is heated most efficiently when the frequency of the RF voltage applied to the tank circuit matches the tank circuit's resonant frequency. That is, when the tank circuit is driven at its resonant frequency the transfer of power from the power supply to the tank circuit is maximized. Because the resonant frequency of the tank circuit changes as the work piece is heated, induction heating systems utilize an RF power supply having a tuning system for continuously tracking the resonant frequency of the tank circuit. By tracking the resonant frequency of the tank circuit, the RF power supply is better able to provide an RF voltage that matches the resonant frequency.
A variety of tank circuits are used in induction heating applications depending on the workpiece to be heated. In some applications it is desirable to have a tank circuit having a relatively low resonant frequency (e.g., 50 KHz), and in other applications a tank circuit having a relatively high resonant frequency (e.g., 1 MHz) is desired. Thus, it is desirable to have an RF power supply that can operate over a broad range of radio frequencies, thereby being able to provide optimum power to a wide variety of tank circuits.
A problem with conventional RF power supplies is that they are only able to provide power to inductive and resistive loads, not to capacitive loads. Total device failure may result if a conventional RF power supply attempts to deliver power to a capacitive load. In an induction heating process the load is the tank circuit in combination with the work piece. When the tank circuit is being driven at its resonance frequency the load appears to be fully-resistive. But, if the tank circuit is not being driven at its resonant frequency, the load may appear to be capacitive, and the RF power supply may experience device failure.
Unfortunately, tuning systems within RF power supplies are not always able to track the resonant frequency of a tank circuit. This is especially true when the resonant frequency is rapidly changing due to heating of the work piece. Thus, there will be times when an RF power supply is not driving a tank circuit at its resonant frequency. Consequently, there are times when the load will appear capacitive. In the situations where the load appears capacitive, the potential for complete device failure within the RF power supply is large. Capacitive loads create reactive energy that if not recovered can overload devices within the RF power supply causing a great deal of damage.
What is therefore desired is an RF power supply that is able to provide power to all types loads, be they inductive, resistive, or capacitive, without experiencing device failure. What is further desired is an RF power supply that is able to operate over a broad-band of radio frequencies so that a wide variety of work pieces can be heated with maximum efficiency.